This invention relates to an extractable barrel-arbor for a watch movement of the type having a first end intended to pivot in a bottom plate, a second end intended to receive a ratchet wheel, a central cylindrical surface for supporting the inner end of a mainspring, and a hook projecting from the central cylindrical surface for fixing the mainspring inner end.
Most of the extractable barrel-arbors known until now, and especially those described in Swiss Pat. Nos. 16,961, 265,249, and 356,724, are solely intended to enable the barrel to pivot between two frame elements of the movement. The core is a separate part which is either secured to the arbor or freely engaged thereon. According to Swiss Pat. No. 265,249, the arbor comprises a central cylindrical bearing surface which is smaller in diameter than the pivot-bearing surfaces, but the former is engaged within the core.
French Disclosed Application No. 2,210,784 shows in FIG. 2 an extractable barrel-arbor, the use of which simplifies manufacture of the movement by dispensing with a separate core. Experience has shown, however, that in this known embodiment, the friction of the last turn of the mainspring is not sufficient to secure the arbor against inopportune axial displacement in the event of severe jolts. Furthermore, this extractable barrel-arbor design necessitates the use of a mainspring having its inner end bent into the shape of a hook, which complicates manufacture.
Another known arbor, disclosed in Swiss Pat. No. 134,172, is intended for hooking a mainspring and is designed so that it can easily be machined out of a round bar. However, the cylindrical support surface extending on each side of the hook is eccentric with respect to the pivoting surfaces. Moreover, the proposed arrangement would not be suitable for a barrel-arbor intended for a watch movement and designed so as to be extractable.
Conventional non-extractable barrel-arbors are machined by turning and milling, the milling operations being intended to form in the cylindrical outer surface of the core a hook which engages in a hole made at the inner end of the mainspring. The last turn of the mainspring, called the eye, rests partly against the milled surface and partly against the outer cylindrical surface of the core; and the two annular shoulders which axially limit these surfaces act, at least in certain cases, as axial stops by cooperating with the barrel-cover and the barrel-drum.
When the barrel is being fitted, the eye must be enlarged by inserting the core, and the spring must be fixed to the hook. Once this has been done, the barrel-cover is set in place, and the arbor can no longer be extracted without dismantling the barrel.
The provision of an extractable barrel-arbor having means for hooking the mainspring necessitates an arrangement which enables the hooking operations to be carried out when the barrel-cover is already in place and when the eye extends freely, approximately at the center of the drum, along a turn having a radius smaller than that of the cylindrical support surface of the arbor.